The present invention relates to lead magnesium niobate perovskite compounds. More particularly, the invention relates to lead magnesium niobate-lead titanate solid solution compounds and products thereof. The present invention also relates to textured lead magnesium niobate-lead titanate solid solution compounds.
Ceramic compounds which have a perovskite crystal structure have numerous commercial applications. These applications include: dielectric materials for capacitors; piezoelectric materials for transducers and sensors; electrostrictive materials for micropositioners and actuator devices; and transparent electrooptic materials for information storage and optical signal processing.
The perovskite structure, as typified by BaTiO3 above 135xc2x0 C., is cubic. This structure is a regular array of oxygen ions at the face centers, small tetravalent titanium ions in the center, and big, divalent barium ions located at the corners. The perovskite structure in ferroelectric compounds is distorted at low temperatures and exhibits tetragonal, orthorhombic, or rhombohedral symmetry. At higher temperatures, the structure transforms to cubic. The transition temperature at which the distorted phase transforms to the cubic phase is called the Curie point. The ferroelectric behavior is caused by distortions in the crystal lattice caused by shifts in the position of the central cation.
A relatively new class of ferroelectric materials is PbO-based complex perovskite corresponding to the formula Pb(B1,B2)O3. The B1 cation can be one of several low valence cations such as Mg2+, Zn2+, Ni2+, and Fe3+, and the B2 cation can be one of several higher valence cations such as Nb5+, Ta5+, and W5+. These ferroelectrics have promise for dielectrics such as ceramic capacitors, piezoelectrics, and electrostrictive actuators (e.g., micropositioner) applications, depending on composition.
Ceramic processing of ferroelectrics of lead magnesium niobate (xe2x80x9cPMNxe2x80x9d) by conventional milling and calcination techniques is difficult. For example, it is extremely difficult to produce PbMg1/3/Nb2/3O3 by conventional mixed oxides processing due to formation of a stable Pb-niobate pyrochlore phase during calcination. Repeated calcination at high temperature (1000xc2x0 C.) is required to form PMN powder. Moreover, at these high temperatures, the volatility of PbO alters stoichiometry and prevents complete reaction. As a result, excess PbO is required.
Several processing steps are required to form a PMN powder into a shape and to densify it into a functional electrical ceramic element. The powder first is formed into a green body such as by dry pressing. The green body then is densified by sintering. Sintering is a key aspect of the manufacturing process and must be controlled to produce uniform, dense ceramic products. The uniformity and density of the products produced, however, are highly dependent on the ceramic powder employed.
Lead-based relaxor ferroelectric-PbTiO3 solid solutions of the perovskite crystal structure which have the general formula Pb(B1B2)O3xe2x80x94PbTiO3, (xe2x80x9cPMN-PTxe2x80x9d)where B1 can be any of Zn, Mg, Sc, Ni, Yb, Fe, Co, Cu, and Cd and B2 is any of Nb, Ta, Ti, Zr, Hf, and W have excellent dielectric and electromechanical properties. Compounds of this formula which are slightly on the rhombohedral side of the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases have excellent dielectric and electromechanical properties. For example, the compound 0.67PMN-0.33PT (67PMN-33PT) which is slightly on the rhombohedral side of the MPB has a longitudinal piezoelectric coefficient (d33) as high as 640-700 pC/N. Also it is known that  less than 001 greater than  oriented cuts of single crystal 65PMN-35PT have piezoelectric coefficients (d33)  greater than 1500 pC/N and longitudinal electromechanical coupling coefficients (k33)  greater than 0.9. See Park et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 44, pp. 1140, 1997.
These properties of PMN-PT type ceramics have spawned renewed interest in growing PMN-PT type ceramics because of their potential for improving the performance of transducers and actuators.
Various methods have been used to grow lead-based ferroelectric single crystals. Typically, these methods employ the high-temperature flux process and the Bridgeman method. These methods, however, have not been satisfactory. For example, the high-temperature flux process has suffered the disadvantage of difficulty of control over the crystallographic growth direction of the single crystals, as well as control of the size of the single crystals produced. In addition, the Bridgman method suffers the disadvantage of poor control over the chemical uniformity of the crystals produced. In addition to these disadvantages, each of these methods requires excess PbO to enhance crystal growth. Excess PbO, however, can limit the properties attainable in the single crystals as well as cause processing difficulties. As a result, control over the chemical uniformity of the PMN-PT crystals is expensive and difficult.
A need therefore exists for a method of manufacture of PMN-PT type textured ceramics which overcome these disadvantages. A need also exists for ceramic powders useful in manufacture of uniform, dense ferroelectrics such as those based on PMN, particularly those based on solid solutions of PMN and lead titanate (xe2x80x9cPTxe2x80x9d).
The present invention relates to PMN compounds, powders and products thereof, especially to PMN-PT compounds, powders and products which have the perovskite structure.
The PMN-PT compounds are characterized by the formula (1xe2x88x92x)Pb(Mg1/3Nb2/3)O3xe2x88x92xPbTiO3 where x may vary from about 0.0 to about 0.95, preferably about 0.0 to about 0.40. More preferably, x is about 0.35. The formula (1xe2x88x92x)Pb(Mg1/3Nb2/3)O3xe2x88x92xPbTiO3 also can be expressed as (1xe2x88x92x)PMNxe2x88x92xPT where x may vary from about 0.0 to about 0.95, preferably about 0.0 to about 0.40. More preferably x is about 0.35.
In a first aspect, the invention relates to preparing lead magnesium niobate-lead titanate compounds of the formula (1xe2x88x92x)Pb(Mg1/3Nb2/3)O3xe2x88x92xPbTiO3 where x=0.0-0.95. The method entails mixing a blend including a source of lead oxide with magnesium niobate and fumed titanium oxide to form a mixture. Examples of useful sources of lead oxide include lead acetates-lead hydroxides such as Pb(CH3COO)2Pb(OH)2, lead acetates such as Pb(CH3COO)4, lead carbonate-hydroxides such as(PbCO3)2Pb(OH)2, and lead carbonates such as PbCO3. The mixture is milled to produce a blend of a particle size less than about 3 xcexcm. Preferably, milling is performed by ball milling in distilled water. The blend is heat treated to produce a dried precursor powder. The dried precursor powder is sintered at about 900xc2x0 C. to about 1300xc2x0 C. to produce a lead magnesium niobate-lead titanate compound.
In a further aspect, the blend may include an oxide of any of Zr, Ta, La, Fe, Mn, Ni, Zn, and W and mixtures thereof. The blend also may include a binder such as polyvinyl alcohol, polyethylene glycol, methylcellulose, carboxymethylcellulose, ethylcellulose, hydroxpropylcellulose, polyethylene oxide base high polymers, acrylic base high polymers, maleic anhydride base high polymers, starch, gelatine, polyoxyethylene alkyl ether, polyvinyl butyrol and waxes.
In another aspect, the invention relates to a process for manufacture of 0.65PMN-0.35PT ceramics such as 0.65PMN-0.35PT single crystals. The process entails mixing (PbCO3)2Pb(OH)2 of a particle size of less than about 6 xcexcm, MgNb2O6 having a specific is surface area of more than about 5 m2/g and fumed TiO2 having a specific surface area of more than about 30 m2/g to form a mixture. The(PbCO3)2Pb(OH)2, fumed TiO2, and MgNb2O6 are present in amounts sufficient to produce a ratio of (PbCO3)2Pb(OH)2:MgNb2O6:fumed TiO2 of about 1:0.24:0.1 to about 1:0.0.27:0.12. The mixture is milled in distilled water to produce a slurry having particle size of less than about 3 xcexcm. The slurry is heat treated to produce a dried precursor powder. The dried precursor powder is ground and sieved to a size less than about 200 xcexcm and compressed to produce a preform. A barium titanate single crystal template is placed on the compressed preform and an additional amount of the dried precursor powder is placed over the barium titanate single crystal. The preform having the barium titanate single crystal and dried precursor powder thereon is compressed to produce a compact. The compact is sintered whereby oriented 0.65PMN-0.35PT ceramics such as 0.65PMN-0.35PT single crystals form on the barium titanate single crystal template. Preferably, sintering is performed at 1150xc2x0 C. in 99% pure oxygen for one hour followed by sintering at 1150xc2x0 C. in nitrogen for ten hours.
This aspect of the invention advantageously enables manufacture of dense PMN-PT compounds without the requirement of the prior art to calcine dried powder and to subsequently re-mill the calcined powder.
This aspect of the invention also advantageously enables manufacture at low temperatures of dense products which have fine microstructures. In addition, this aspect of the invention advantageously enables manufacture of PMN-PT solid solution compounds without the need to use excess PbO as in the prior art.
In another aspect of the invention, templated grain growth (TGG) using {001} SrTiO3 single crystal templates are employed to produce textured, PMN(1xe2x88x92x)-PTx ceramics where x=0xe2x88x921, preferably, about 0.325 to 0.35, most preferably about 0.325, in directions such as, the  less than 001 greater than  direction.
TGG entails growing oriented PMN-PT single crystals onto {001} SrTiO3 single crystal templates dispersed within a PMN-PT precursor matrix. The amount of the {001} SrTiO3 templates in the PMN-PT precursor matrix may vary from about 1 vol % to about 10 vol %, preferably about 5 vol % based on the volume of PMN-PT ceramic product produced. The size of the {001} SrTiO3 templates employed may vary from about 1 xcexcm to about 50 xcexcm in edge length, preferably about 5 xcexcm to about 25 xcexcm in edge length. The aspect ratio of length to thickness of the {001} SrTiO3 templates may vary from about 2 to 100, preferably about 3 to about 30, most preferably about 5 to 20.
This aspect of the invention advantageously enables use of {001} SrTiO3 templates and a PMN-PT precursor matrix to produce textured PMN-PT ceramics such as  less than 001 greater than  textured PMN-PT ceramics.
The densities of the  less than 001 greater than  textured PMN-PT ceramics produced by this aspect of the invention advantageously are  greater than 98%, preferably  greater than 99% of theoretical density.
Having summarized the invention, the invention will now be described in detail by reference to the following detailed description and non-limiting examples.
In manufacture of PMN-PT compounds, a source of lead oxide is mixed with fumed TiO2 and MgNb2O6 to produce a blend. Examples of sources of lead oxide include but are not limited to lead acetates-lead hydroxides, lead acetates, lead hydroxides, and lead carbonates. Lead acetates-lead hydroxides may include Pb(CH3COO)2Pb(OH)2; lead acetates may include Pb(CH3COO)4; lead hydroxides may include Pb(OH)2. Preferably, the source of lead oxide is (PbCO3)2Pb(OH)2.
The (PbCO3)2Pb(OH)2 is mixed with fumed TiO2 and MgNb2O6 to produce a blend. The blend may be milled such as by jet milling or ball milling. Preferably, the blend is ball milled in the presence of a liquid to produce a slurry of particles and liquid. More preferably, ball milling is performed for about 1 h to about 10 hours. The purity of the (PbCO3)2Pb(OH)2 employed may vary from about 98% to about 99.99% pure, preferably about 99% to about 99.9% pure, most preferably about 99.9% pure. The particle size of the (PbCO3)2Pb(OH)2 can be less than about 6 xcexcm, preferably less than about 5 xcexcm, more preferably less than about 4 xcexcm. The specific surface area (xe2x80x9cSSAxe2x80x9d) of the fumed TiO2 can be more than about 30 m2/g, preferably more than about 40 m2/g, more preferably more than about 50 m2/g. The SSA of the MgNb2O6 can be more than about 5 m2/g, preferably more than about 6 m2/g, more preferably about 7.5 m2/g. The ratios of amounts of (PbCO3)2Pb(OH)2, fumed TiO2 and MgNb2O6 for manufacture of 0.65PMN-0.35PT can vary. Examples of useful ratios of amounts of (PbCO3)2Pb(OH)2:MgNb2O6:fumed TiO2 are about 1:0.24:0.1 to about 1:0.0.27:0.12, preferably about 1:0.25:0.1 to about 1:0.26:0.12, more preferably about 1:0.256:0.109.
Various liquids may be used in ball milling. Examples of useful liquids include alcohols such as ethyl alcohol, isopropyl alcohol, acetone, deionized water, and distilled water, preferably distilled water and deionized water. Examples of milling balls which may be employed include yittria stabilized zirconia balls and alumina balls.
The weight ratio of liquid to particles in the slurry can vary from about 1:0.5 to about 1:0.32. Preferably the weight ratio of liquid to particles in the slurry is about 1:0.32.
Ball milling of the mixture of (PbCO3)2Pb(OH)2, fumed TiO2 and MgNb2O6 is continued to produce a slurry which has a particle size less than about 3 xcexcm, preferably less than about 2 xcexcm, more preferably less than about 1 xcexcm in size.
After ball milling, the slurry is heated at about 50xc2x0 C. to about 120xc2x0 C., preferably about 60xc2x0 C. to about 100xc2x0 C., more preferably about 70xc2x0 C. to about 90xc2x0 C., most preferably about 80xc2x0 C., with stirring to produce a dried PMN-PT precursor powder. The dried precursor powder is ground and sieved to less than about 200 xcexcm, preferably less than about 150 xcexcm, more preferably less than about 90 xcexcm. The dried precursor powder is compressed by uniaxial or isostatic pressure to produce a compact. The compact then is isostatically compressed to produce a green preform. Uniaxial pressing may be done at about 5 MPa to about 100 MPa, preferably about 5 MPa to about 50 MPa, more preferably about 5 MPa to about 20 MPa. Isostatic pressing may be done at about 100 MPa to about 400 MPa, preferably about 100 MPa to about 350 MPa, more preferably about 100 MPa to about 300 Mpa.
The green preform then is sintered. During sintering, the green preform is encapsulated in Nobel metal foil such as Pt and placed into an embedding powder. Embedding powders which may be used include lead containing powders such as lead oxide, lead magnesium niobate, and lead zirconium niobate. These powders are capable of surrounding the preform with an atmosphere of lead oxide during sintering. In manufacture of PMN-PT compounds, the embedding powder preferably has about 1% more PbO than the PMN-PT composition of the green preform. More preferably, the powder has the same composition as the green preform.
The embedding powder preferably has a composition identical to the green preform to provide an atmosphere of lead oxide around the green preform. Where the embedding powder has a composition identical to that of the green preform, encapsulation of the green preform in the noble metal foil is optional.
The green preform can be sintered in oxygen, nitrogen, or air. In a preferred aspect, the preform is sintered in oxygen, preferably 95% pure oxygen, more preferably 99% pure oxygen. In another preferred aspect, the preform is sintered in oxygen and then in nitrogen. During sintering, the green preform may be heated at about 3xc2x0 C. to about 20xc2x0 C./min, preferably about 5xc2x0 C. to about 15xc2x0 C./min, more preferably about 10xc2x0 C. to about 15xc2x0 C./min, most preferably about 15xc2x0 C./min.
Sintering temperatures can vary from about 900xc2x0 C. to about 1300xc2x0 C., preferably about 1000xc2x0 C. to about 1200xc2x0 C., more preferably about 1000xc2x0 C. to 1150xc2x0 C. The time periods at which the preform is held at the sintering temperature can be up to about 50 hours, preferably about 0.5 to about 20 hours, more preferably about 1 to about 10 hours.